Improving system throughput and capacity is one of the main goals for high-speed wireless systems, such as those proposed for the High Speed Downlink Packet Access (HSDPA) and 1xEV-DV which are part of the 3rd Generation Partnership Project (3GPP) and 3GPP2 standards bodies, respectively. Technologies, such as adaptive modulation and coding schemes, Hybrid-ARQ, code combining, smart antennas, etc., are being investigated and proposed for these new standards to achieve this goal.
ARQ is an implicit adaptation technique in which receiver acknowledgments regarding the transmitted packets are used for re-transmission decisions. Hybrid-ARQ (H-ARQ) is one form of ARQ schemes in which one of code combing methods is usually used. Time diversity offered by Hybrid ARQ has been shown to have the capability of improving system throughput. In the 1xEV-DV and HSDPA proposals, for example, Chase combining (i.e. symbol combining, also referred to as H-ARQ-type-III with one redundancy version) and Incremental Redundancy (IR), are used together with ARQ to achieve higher throughput on the forward link by exploring this time diversity. Transmit Diversity (TD) has been proposed for wireless standards such as the IS-2000 and Wideband Code Division Multiple Access (WCDMA). In IS-2000, for example, two open loop transmit diversity schemes have been adopted, i.e. Orthogonal Transmit Diversity (OTD) and Space Time Spreading (STS). In WCDMA, both open loop TD, i.e. Space Time Transmit Diversity (STTD), and closed loop TD schemes have been adopted. In the HSDPA and the 1xEV-DV proposals, different open loop and closed loop transmit diversity schemes are under consideration.
Closed loop transmit diversity requires feedback information from the receiver to achieve better performance compared to open loop transmit diversity. However, the quantized feedback information with reasonable degradation introduces unwanted overhead. Open loop TD can be considered as special case of closed loop TD in which feedback is a constant value. Many open and closed loop transmit diversity schemes have been studied. For example, a four transmit antenna system of the double-STTD or (DSTTD) scheme has been proposed where two distinct streams of data are encoded using two STTD encoders and transmitted by 4 antennas. A closed loop TD scheme with a four transmit antenna system of the DSTTD has also been proposed where an estimation of the co-variance matrix is required in order to find the best basis and the feedback is required to signal to the transmitter which basis would be used. A discussion of a prior art DSTTD system can be found in an article entitled, “Increasing data rate over wireless channels”, by A. Naguib, N. Seshadri and A. R. Calderbank, IEEE Signal Processing, pp. 76-92, May 2000.